The direct synthesis of methylchlorosilanes is carried out by reacting silicon and methyl chloride at 250.degree. to 300.degree. C. in the presence of copper catalysts. In addition to the silanes of the general formula Me.sub.x SiCl.sub.4-x, in which x has a value from 0 to 4 and Me here and below denotes a methyl group, small amounts of ethyl-chlorosilanes, various hydrodosilanes, above all Me.sub.y HSiCl.sub.3-y, in which y has a value from 0 to 2, and ethyldichlorosilane EtHSiCl.sub.2 are also formed. Various straight- and branched-chain alkanes, alkenes and chlorohydrocarbons having up to 9 carbon atoms appear as impurities. The direct synthesis is described, inter alia, in W. Noll, Chemistry and Technology of Silicones, Academic Press, Inc., Orlando, Fla., 1968, chapter 2.2.
The most sought-after target product of the direct synthesis is Me.sub.2 SiCl.sub.2, which can be converted by hydrolysis and polycondensation into silicone polymers having diverse functional groups and structures.
An essential quality feature of most silicone polymers is as low a content as possible of trifunctional impurities in the polymer skeleton. One of the possible trifunctional impurities of the Me.sub.2 SiCl.sub.2 employed is EtHSiCl.sub.2.
Since the boiling points of Me.sub.2 SiCl.sub.2 (70.degree.-71.degree. C.) and EtHSiCl.sub.2 (74.degree.-76.degree. C.) differ from one another by only about 4.degree. C., a very distillative effort, such as high reflux ratios, a large number of theoretical plates, and trays in practice, a high energy requirement and a reduced space/time yield, is necessary to obtain the Me.sub.2 SiCl.sub.2 in the purity required for the particular use.
EP-A 423,948 describes the conversion of hydrogen-containing alkylsilane impurities into the corresponding alkylchlorosilanes using hydrogen chloride gas and suitable catalysts of Pd, Pt, Rh, Ru, Ni, Os, Ir and compounds thereof. The difference in boiling points between the target product and impurity is increased by this measure such that the distillation can be operated with a considerably reduced effort.
The disadvantage of this process is that hydrogen chloride additionally has to be introduced into the silane product stream in an amount of hydrogen chloride above the stoichiometrically required amount must be employed. The excess portions of hydrogen chloride interfere in the subsequent distillation and therefore have to be removed beforehand. Another disadvantage of the process described is that at an original H-silane concentration in the range from 10 ppm to 10%, a contact time of more than 1 minute is preferably for adequate conversion of the hydrogen-containing silane. The reaction of chlorohydrocarbons with halogen-free hydrogen-containing silanes in the presence of noble metal catalysts to form hydrocarbons and chlorosilanes is known, for example, from D. J. Citron, J. E. Lyons; L. H. Sommer, (The Journal of Organic Chemistry, 1969, volume 34, page 638). In this case, the degrees of conversion depend very greatly on the individual components and the catalyst.
U.S. Pat. No. 4,774,347 describes a process for reducing the chlorohydrocarbon content in silane streams in the presence of hydrogencontaining silanes with the aid of catalysts which form Lewis acids. Aluminum, aluminum silicates, zeolites, aluminum chloride, cobalt chloride, iron chloride, copper chloride, tin chloride, palladium chloride or zirconium chloride are employed as catalysts in this process.
The disadvantage of this process is that the halogen-containing catalysts dissolve in the methylchlorosilanes, as a function of the temperature, and that the oxides and metals mentioned are in some cases converted into the corresponding chlorine compounds and thereby into a soluble form. This means that the catalysts mentioned can be employed in a continuous reaction in the liquid phase carried out industrially to only a limited extent, since the catalyst is dissolved off the support material and removed from the reaction system with the methylchlorosilane stream. The service life of the catalyst is thereby greatly reduced. In addition, the silanes are contaminated with metal halides. Another disadvantage of the process is that a contact time of the components on the catalyst of more than one minute is necessary for corresponding degrees of conversion.